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Site History . Jackpot Food Mart, 1434 W Yosemite Ave , Manteca, CA Time Oil Property No 04-167 PHS/EHD and CRWQCB Authorization to proceed with the proposed work was received from the PHS/EHD on August 4, 1992 On August 27, 1992, a soil vapor survey was performed to assist in determining the vertical and lateral extent of contamination at the subject site A pneumatic hammer was utilized to drive 9 soil vapor probes to depths of up to 15 feet in the vicinity of the former tanks Vacuum pumps and charcoal tubes were used to collect vapor samples from each probe at various depths Immediately after collection, each sample was submitted to an onsite mobile lab where it was extracted and injected into a gas chromatograph The results of the survey suggested that impacted soils were limited to the area of the north pump island and did not extend laterally to any great degree A copy of the Canonie's "Soil Vapor Survey Report with Proposed Boring and Well Locations" was forwarded to the PHS/EHD and CRWQCB on September 24, 1992 During the week of October 5, 1992, a drill rig was utilized to install 4 groundwater monitoring wells and 3 vadose zone vapor extraction wells in the vicinity of the former tanks During field activities, soils were found to consist of loose fine to medium sands to a depth of approximately 15 feet This strata was underlain by a 1-foot to 4-foot thick layer of stiff silty clay which • appeared to act as an aquitard (15 to 19 feet below ground surface) Just below the clay unit, saturated sands/silty sands were encountered, while moist silty clays were encountered at 30 feet Wells Wl, W2, and W4 were constructed as vadose zone extraction wells Each well was installed to a depth 15 feet and screened from a depth of 5 feet to the extent of the well with 4-inch diameter, 0 020-inch slotted well screen Wells W3, W5, W6, and W7 were installed as groundwater monitoring wells Because it was thought that the first clay unit represented a possible aquitard, these wells were constructed to insure that contamination to the groundwater did not occur through the creation of a vertical conduit Accordingly, W5, W6, and W7 were completed as 30-foot monitoring wells, screened from a depth of 20 feet to the extent of the well with 2-inch diameter 0 020-inch slotted casing Due to its potential for use in a pumping system, W3 was constructed with 4-inch diameter casing and screened similarly to the 2-inch wells Two to three soil samples from each boring were transported to an analytical laboratory and analyzed for TPH, BTEX, and ethylene dibromide Gasoline and xylenes were indicated at 100 ppm and 2 ppm, respectively, in the sample collected from W2 at a depth of 5 feet, however, no contaminants were indicated at a depth of 15 feet Samples collected from W4 at a depths of 5 feet and 10 feet contained 26 ppm and 290 ppm gasoline, as well as ethylbenzene and xylenes However, soil samples collected from W3 (installed adjacent to W4) contained only 0 71 ppm gasoline and trace BTEX constituents at a depth of 14 feet, while no contaminants were found at 20 feet None of the other samples contained detectable concentrations of gasoline or BTEX • On November 5, the groundwater wells were developed and sampled Prior to purging, an east-southeasterly gradient was determined by measuring the depth to 2